The present invention relates generally to forming fiber reinforced plastic preforms and, more particularly, to a method and apparatus for binding the fibers of a fiber reinforced plastic preform.
Fiber reinforced plastic (FRP) parts or composite parts are well known and used in a wide variety of applications. An FRP part generally consists of a plastic shape in which carbon, fiberglass, or other reinforcing fibers are dispersed in order to provide strength to the plastic. One method of making an FRP part is known as resin transfer molding (RTM). In RTM, fibrous material in a mold is injected with resin which cures to form the part. Examples of these techniques are disclosed in commonly assigned U.S. Pat. Nos. 4,740,346; 4,849,147; and 4,863,771, which are hereby incorporated by reference. In RTM, fibrous material is often formed into a preliminary shape before being placed into the mold. The shaped sections generally conform to the shape of adjacent mold die surfaces and are known as preforms. Preforms have been made in several different manners. One approach is to direct chopped fibers by means of a flow of air onto a screen. One problem with this technique is that it is difficult to obtain desired fiber orientation. Another method is to make the preforms from mats of fibrous material. This method however results in undesirable amounts of scrap and is labor intensive thus resulting in production cost inefficiencies. Still another technique known as a wet slurry process is disclosed, for example, in Keown et al., "Wet Slurry Process Brings Precision To Reinforced Plastics". As discussed therein, a slurry containing chopped glass strands or fibers is sucked by vacuum into a chamber covered by a screen. As a result the fibers are deposited on the screen. This approach, however, also has some drawbacks. For example, it is somewhat difficult to consistently obtain the desired fiber orientation and compactness or density of the fibers using this equipment. In addition, the pumps and other equipment required to create the vacuum and suck the slurry through the screen may be unduly complex and difficult to maintain. Furthermore, the process is relatively slow. An improved wet slurry process is disclosed in commonly assigned U.S. Pat. No. 5,039,465, which is also hereby incorporated by reference. The process disclosed therein teaches drawing the slurry through the screen by raising the screen through a tank containing a slurry of fibers resulting in the fibers being deposited on the screen. Further disclosed are unique configurations for the holes in the screen to direct the fibers into desired orientations.
Of concern in any preform forming process is handling the fiber preform without disturbing the shape of the preform or the orientation of the fibers. To this end, it is known that a binding material may be deposited along with the fibers when forming the preform. The binding material gives the preform sufficient strength to allow for handling without disturbing its shape or the orientation of the fibers. In typical applications, the binding material is a thermoplastic fiber mixed into the slurry with the reinforcing fibers as in a wet slurry process or blown along with the reinforcing fibers as in an air blown process onto the screen. After the preform is shaped, the preform, still on the screen, is placed into an oven where it is heated thus melting the thermoplastic fibers and binding the reinforcing fibers. Once cooled, the preform can be removed from the screen and handled without damaging it. However, this heating and cooling process, often taking several hours, is not well suited for mass production in that it requires the use of many screens, large ovens, and is very time consuming.